While you are right about most, if not all, of everything here, Ross is no longer captaining this team. He is a play the vets manager, and Counsell isn't sold on the same idea. I'm not saying they will now hand the keys to a certain position to Mervis, but he should get a better opportunity to provide his ability to this team. I could see him start the year in Iowa, or as an extra bat off the bench/spot starter at the big league level. A lot of where he starts could depend entirely on how good of a Spring he has.
I don't think they should be closing the door to Morel or Busch at 3rd base, since you could afford a bit of weaker defense at 3rd for 6 innings (Madrigal can be a defensive replacement) for the extra bat early in games. Morel is a righty bat at DH. Mervis is a lefty bat at DH. While a full on platoon might not make sense, you might want Morel to have 66% of the plate time at DH if that's the position he's destined, then 33% for Mervis vs. tough righties is still a nice amount of plate time. Then if you can get one of Busch or Morel to be a decent enough 3b for half a game or more, Mervis' ability to play 1b and provide another lefty bat could be quite valuable.
I'd rather see them give Mervis a crack at the roster than Wisdom. But, I'm also still in the camp of sign Chapman and Bellinger. And yes, I know that blocks a lot of prospects. Trade some of those prospects at the deadline for pitching and a strong playoff run.